This invention relates to simple, low cost electronic scales in general and more particularly to an improved low profile bathroom scale utilizing a planar coil displacement transducer.
Most present day bathroom scales use inexpensive lever systems to convert weight into vertical motion which, in turn, is converted into horizontal or rotary motion. In electronic bathroom scales the vertical or rotary motion is converted by means of incremental encoders or position sensors to provide an electrical signal.
As indicated above, most of the present day bathroom scales which are relatively inexpensive are based on a flexure arrangement which essentially collects the weight on the platform as distributed in the four corners and converts to one simple linear deflection which works against a spring. In conventional mechanical scales this deflection is converted into a large rotary motion by means of a gear and rod and the weight is indicated by means of a rotatable disk which has weight markings about the periphery. In the electronic version, the disk is transparent, the lines are etched on it, and the motion is picked up by an optical sensor.
In other electronic bathroom scales the vertical motion is coverted into a relatively large horizontal displacement which is sensed by a LVDT and then converted into a digital signal. Essentially, as above indicated, most scales do not operate to sense the vertical motion of the platform but use relatively crude conversion into a large horizontal motion or into a large rotary motion. The requirement to convert to large horizontal displacements (such as 0.5") or large rotary motion (such as 330.degree.) dictate that the mechanism must be very compliant and therefore subject to errors such as zero drift and non-repeatability.
It is possible to use a pressure sensor or a strain gage which is mounted on a cantilever beam to provide an output voltage proportional to the weight placed on the platform. This output voltage may be converted to a frequency or otherwise employed to activate an electronic circuit to thereby produce a digital output indicating the magnitude of the weight. However, these approaches are relatively costly. A further restriction is that the motion sensor must be relatively low profile as bathroom scales must be relatively flat for the convenience of the user.
In the present invention, there is provided a means for sensing the vertical motion directly using a low profile displacement transducer which is also low-cost and suitable for mass-production. To be defined as low profile, a displacement transducer used to sense horizontal motion in a bathroom scale, must meet the following requirements:
1. It must be able to detect small vertical deflection such as 0.050 inch to 0.100 inch. PA0 2. It must not be substantially higher than the displacement it measures. PA0 1. It must be produced using a batch process technique. PA0 2. It must have a low parts count. PA0 3. It must be easy to assemble.
To be defined as suitable for mass-production, a transducer must meet the following requirements:
As will be explained, the displacement transducer in this invention meets all the requirements. Further, the arrangement simplifies the bathroom scale mechanism by eliminating the need for a motion conversion such as vertical to horizontal or vertical to rotary. As will be further explained, the apparatus of the present invention allows one to detect small vertical motions, enabling to make the mechanism much stiffer and more accurate. As one will understand, it is a major objective of the present invention to provide a low profile electronic scale apparatus which employs a low cost reliable and repeatable displacement sensor device whose output is converted by means of electronic circuitry to an accurate digital readout of the weight.